Calcium is an ubiquitous element that occurs commonly in nature both in soluble (calcium chloride, sea water) and insoluble (gypsum, limestone) forms. It is a macronutrient essential to all life forms. Certain soluble salts of calcium have large scale commercial significance in agriculture, road stabilization, as a soil amendment, as food additives, in the treatment of oily wastes, and as desiccants for drying gases and liquids.
Gypsum, a calcium sulfate, and limestone, a calcium carbonate, have been used to improve soil quality, but have relatively low solubility and therefore act slowly as soil conditioners. The soluble calcium salts, including calcium chloride which is the most prominent, present problems: liquid forms of these salts create bulk for shipping and storage purposes because of their high water content (65% to 70% H.sub.2 O). Anhydrous soluble calcium salts (90% to 95% CaCl.sub.2), however, are hygroscopic and deliquescent, i.e. they absorb water to the point that the salts dissolve, creating even more long term shipping, storage and handling problems.
The use of calcium in soil conditioning and other industries has grown significantly over the past decade despite these problems. Solutions containing calcium ions are known to greatly improve hydraulic conductivity and water infiltration rates when applied to soils containing clays which have been swollen by sodium absorption. The addition of other nutrients such as nitrogen, phosphorus, potassium, magnesium, iron, zinc, copper, manganese, molybdenum, boron and sulfur, is a good fertilization technique to improve the yield of crops.
Commercial solutions of calcium chloride and urea, the urea added to provide nitrogen in the conditioning process, are also available. Disadvantages of these solution products include the high water content which increases transportation and storage costs per unit of calcium, and the requirement that liquid application devices be used. Moreover, dry granular conditioners are more compatible with the availability of equipment currently used in agriculture and land-use industries to spread other dry soil additives and/or fertilizers. Also, for shipping and storage purposes, anhydrous salts are obviously preferred because of ease of handling and cost effectiveness. If necessary, they may be returned to a soluble state by mixing in water on site.
As mentioned above, the hygroscopic and deliquescent properties associated with the anhydrous soluble calcium salts and some of their hydrates create storage, handling and usage problems. Frequently, moisture is absorbed by the stored salt and converts the initially free-flowing salt to a solid rock-like mass. This severely limits the shelf life, creates handling and disposal problems and generally leads to damaged goods expenses, and sometimes total loss of product.
What is needed is a dry, granular soluble calcium-containing product in which the moisture-absorbing characteristics associated with the highly-soluble calcium salts are significantly inhibited. Further, a calcium-containing material which contains a soluble calcium salt coated with a nitrogen containing material and/or other nutrient materials would both inhibit moisture uptake and meet the requirements of being compatible with widely available spreading equipment. This simultaneous application of soil conditioning calcium with other required macronutrients and/or micronutrients would have significant application to the agricultural industry. Unfortunately, admixtures of hygroscopic calcium salts with plant nutrients do not avoid the moisture uptake problem. For example, calcium chloride and urea form an adduct that is at least as or even more hygroscopic than calcium chloride itself.
Prior to the present invention, the treatments available to improve characteristics of fertilizer or calcium containing products include (1) fertilizers or other materials coated with a hydrophobic material, usually to prevent dusting or flaking; and (2) admixtures of calcium containing products with additives for improved qualities, often dust control.
Whittaker et al. disclose in U.S. Pat. No. 2,074,880 a molecular addition compound of calcium sulfate and urea for use as fertilizer. Muntzer et al. in U.S. Pat. No. 3,804,661 disclose a method for producing a solid material having hydrophobic and oleophilic characteristics comprising particles of chromium salt coated with paraffin used for cleaning up oil spills. U.S. Pat. No. 3,906,140 to Capes discloses co-pellets of calcium chloride and sodium chloride formed by using binder liquid salt solutions that form bridges between the initial particles upon drying. These references do not disclose the prevention of moisture absorption by coating core materials.
U.S. Pat. No. 1,592,971 to Dow discloses particles of calcium chloride in one state of hydration coated by particles of calcium chloride in a lower state of hydration. Buchholz et al. in U.S. Pat. No. 5,360,465 disclose particulate fertilizer dust control by treating inorganic fertilizers with a solution of urea and lignosulfate. The inorganic fertilizers include limestone and dolomite. It is noted that limestone is a form of calcium carbonate, and dolomite is a form of a calcium magnesium carbonate. Neither are hygroscopic or deliquescent. Gleckler et al. in U.S. Pat. No. 4,252,831 disclose feed grade calcium phosphates coated with molasses and surfactant used to minimize dusting. Buchholz and Glecker both attempted to reduce dusting.
Campbell et al. in U.S. Pat. No. 3,331,677 disclose a method of preparing particulate fertilizer that is coated with a urea-wax adduct. Each particle contains a core which is overlaid with a thin film which substantially covers the surface of the core. The core is a dispersion of urea in paraffin wax. Both Buchholz and Campbell maximized the use of urea in fertilizer by mixing it with additives. Campbell taught that coating fertilizer with a urea-wax adduct may prevent caking. Zaayenga in U.S. Pat. No. 3,192,031 discloses coated fertilizer compositions comprised of urea coated with a first coating of diatomaceous earth and a second coating of wax.
Buhler et al. in U.S. Pat. No. 3,740,248 disclose starch particles coated with wax to prevent offset of print from freshly printed sheets. Dubois in U.S. Pat. No. 4,012,537 discloses de-icing compositions contained in asphalt road surface material comprised of calcium or sodium chloride coated with linseed oil or other water-tight coating that may be worn away by mechanical action such as traffic movement.
None of these references, however, teach or suggest a core of a calcium-containing material which is hygroscopic or deliquescent combined with a less hygroscopic, soluble non-calcium coating material to inhibit the uptake of moisture by the core material.